An image sensor device is an integrated circuit (IC) having an array of pixels and other circuitry and devices for sampling the pixels, outputting the sample values and processing the sample values. One problem that arises in image sensor ICs is that power supply noise results in temporal row-wise noise in the imaging pixels. One technique for suppressing the row-wise noise is to include reference columns of dark, non-imaging pixels in the imaging device that are used to remove noise from the imaging pixels. The “dark” pixels are covered with a metal layer that prevents light from impinging on the photodiodes of the pixels. Thus, the dark pixels are non-imaging pixels. The pixels that are not covered by the metal layer and that receive light are referred to as imaging pixels. By taking the difference between the values of the dark pixels and the values of the imaging pixels, temporal row-wise noise that results from power supply noise can be cancelled.
FIG. 1 illustrates a block diagram of a known image sensor device 2 having an imaging array 3 and a non-imaging array 4. The imaging array 3 is an array of pixels 5 that are uncovered and therefore receive light. The non-imaging array 4 is an array of dark pixels 6 that are covered by a metal layer (not shown), and therefore do not receive light. The rows of pixels 5 of the imaging array 3 and the rows of pixels 6 of the non-imaging reference array 4 use the same transfer and reset control lines TX and RST, respectively. Additionally, row sampling circuit 7 and reference row sampling circuit 9 use the same sampling signals, S1 and S2, (not shown) to sample the corresponding reset and video signals from the pixels. Accordingly, whenever the reset and video signals from a pixel in a row of the imaging array 3 are sampled (the difference between the reset sample and the video sample forming a pixel sample), corresponding reset and video signals from the same row in the non-imaging array 4 are sampled and used to cancel out row-wise noise from the pixel samples of the imaging array 3. For ease of discussion, the difference between the reset sample and the video sample values for a given pixel in the imaging array will be referred to herein as a pixel sample value. Likewise, the difference between the reset and video sample values for a given pixel in the non-imaging array 4 will be referred herein to as a dark pixel sample value.
For example, when the pixels 5 from Row M-1 of the imaging array 3 are sampled, the dark pixels 6 from Row M-1 of the non-imaging reference array 4 are also sampled. The pixel sample values of Row M-1 of the imaging array 3 are output on vertical route lines 11. The pixel sample values of Row M-1 of the non-imaging array 4 are output on vertical route lines 15. The row sampling circuit 7 samples and holds pixel reset and video signals that are presented on vertical route lines 11 of the imaging array 3. Reference row sampling circuit 9 samples and holds dark pixel reset and video signals that are presented on vertical route lines 15 of the non-imaging reference array 4. An average of dark pixel sample values is generated for each row in the non-imaging averaging circuit 17. The sample selection circuit 14 selects the pixel sample values in a sequential order starting with Col 1 and ending with Col N. As the pixel sample values are selected, they are provided to a difference circuit 8. The average value of the dark pixel sample values for the selected row in the non-imaging array is provided by the averaging circuit 17 to the difference circuit 8. The difference circuit 8 then subtracts the average dark pixel sample value for the samples received from the non-imaging array 4 from each of the pixel sample values received from the imaging array 3 for the same row to produce a final output value. The final output value is a sample value from which row-wise noise has been cancelled.
One problem associated with the technique described above with reference to FIG. 1 is that it is possible for one or more rows of the non-imaging array 4 to have an offset or variation in it due to factors such as process variations or defects, unmatched coupling of control signals into sample paths, power supply noise, etc. For example, an offset may result in a row of the non-imaging array 4 having a pixel in it that is too bright, which is often referred to as a “hot” pixel. When these offsets or variations exist in a particular row of the non-imaging array 4, using the sample average value for that row to remove noise will often result in artifacts being present in the final output image. These artifacts may make a row appear too bright or too dim.
Another problem associated with the technique described above with reference to FIG. 1 is that covering the non-imaging array 4 with metal creates a mismatch between the parasitic capacitance of the rows and columns of the non-imaging array 4 and the parasitic capacitance of the rows and columns of the imaging array 3. This mismatch in parasitic capacitance prevents perfect cancellation of row-wise noise during the pixel sampling operations.
Accordingly, a need exists for a method and apparatus for more effectively removing row-wise noise from pixel samples of an imaging array.